1. Field of the Invention
This invention relates to a measurement and control system for scanning sensors, and more particularly, to an accurate measurement and control system which allows a sensor to be scanned with respect to any arbitrarily selected reference coordinate system.
2. Discussion
Precision measurement and control systems for scanning sensors are used in a wide variety of applications in which detection of objects is needed. These systems provide for the positioning and scanning of a sensor along a predefined scan path. The sensor is capable of detecting any object coming into its field of view. The sensor may be moved along this scan path at a variety of angular rates. High angular rates are desirable, particularly at "turnaround" in which the sensor has completed its scan path and must be quickly returned to the initial scanning point.
Previous measurement and control systems for scanning sensors were generally limited to scanning the sensor relative to base referenced or inertial coordinate systems. These previous systems were typically implemented in one of two ways. The first implementation employed rate integrating gyroscopes, one of which was mounted to the scanned object such that its input axis was along the desired scan axis. An electronic signal, proportional to the desired scan rate, was applied to torquers which were mounted on the gyroscope. The output of the gyroscope was proportional to the error between the desired scan rate and the actual scan rate. This error signal was then used as an input to a rate control loop which provided torque commands to the torquers to adjust for the error. Two or three rate integrating gyroscopes could be mounted orthogonally on the sensor in order to cause the sensor to scan in two or three dimensions.
This implementation had three major limitations associated with it. The gyroscopes produced outputs if they were moved about any axis, not just the desired scan axis. These outputs, associated with cross-axis coupling, introduced error into the system by producing incorrect scan rate error signals. Secondly, during times of high angular acceleration of the sensor, the output signals of the gyroscopes weren't representative of the gyroscopic motion. These output signals were in error and produced corresponding system errors. Lastly, the sensor could only be scanned relative to an inertial coordinate system.
The second implementation utilized a precision relative angular measuring device such as a resolver, an inductosyn, or a shaft encoder. These devices were used to measure the position of a rotor shaft relative to its case. The output signal of these devices was subtracted from a position command, which was previously input to the sensor, thereby producing an error signal. This error signal was then used in a position control loop to control the position of the sensor. The difficulty associated with this implementation was that the mechanical compliance between the controlled shaft axis and the sensor line of sight limited the accuracy of these systems since the quantity that was measured (i.e. shaft angle) was not the quantity that was desired to be controlled (i.e. the angle that the sensor makes with respect to its base). Furthermore, normally this implementation could only scan relative to a coordinate system which was referenced to the base of the sensor. However, if one knew the orientation of the sensor's base with respect to a selected reference coordinate system, the sensor, in this implementation, could be scanned relative to this arbitrary reference coordinate system.